mmm god is less of a word and more of an expression. it is kind of hard to explain, but the origins are from hepper, huckins, solberg, moore, montez, and a few others as far back as sixth grade.. at first it was nothing more than a joke, but quickly grew into widespread usage... you can use it as a rejection of a request (its most common use) or an interjection

example A: (rejection)

Teacher: Eric, Will you help me move these books?
Eric: mmmgod la weez